The cellular expression of immune response (Ir) function has been studied in an in vitro system of primary antibody responses to the TNP conjugates of (T,G)-A- -L and (H,G)-A- -L. These responses require the participation of T cells and accessory cells as well as B cells, and are under control of genes mapped to the K or I-A subregions of the H-2 complex. It has been demonstrated that the function of the accessory cells in responses to TNP-(T,G)-A- -L and TNP-(H,G)-A- -L is under the control of genes which also map to K or I-A. In contrast, both B cells and T cells from nonresponder H-2a strains are competent in supporting primary antibody responses to TNP-(T,G)-A- -L, and therefore do not appear to express the Ir gene defect present in these strains.